The present invention relates to apparatus and to a method for measuring color.
To measure the color of an opaque object, it is known to use a colorimeter comprising a light source for illuminating the object, optical analyzer apparatus for analyzing the light reflected by the object, and calculation means responsive to the signals delivered by the optical analyzer apparatus to determine the tristimulus values X, "Ugr", and Z of the color of the object in a reference colorimetry system, for example the system adopted in 1931 by the International Commission on Illumination (CIE).
The optical analyzer apparatus includes means for splitting the light reflected by the object into three beams, each of which passes through a filter system and terminates on an associated photoelectric cell.
That type of known calorimeter suffers from the drawback of requiring direct contact between the optical analyzer apparatus and the object whose color is to be measured, and of being unsuited to measuring the color of an object remotely or to measuring the color of an object that is not opaque, that diffuses or absorbs light while also passing a portion thereof, such as the skin or certain plastics or kinds of makeup.
To mitigate those drawbacks, attempts have been made to measure color by means of a video camera.
Nevertheless, so far as the Applicant company is aware, there still does not exist any apparatus which is suitable for remotely measuring the color of an object, in particular an object that is not opaque, and which is capable of measuring color accurately, reliably, and quickly while nevertheless being of relatively low price.
A particular object of the present invention is to provide novel apparatus for measuring the color of any type of object, using a video camera, and making it possible to measure color accurately, reliably, and quickly while being of relatively low cost and easy to use.
The invention achieves this by means of an apparatus for measuring color, comprising:
an acquisition system including a video camera, said acquisition system being suitable for delivering signals representative of the tristimulus values in a colorimetric system associated with said acquisition system for the color of an object placed in the observation field of the camera; and
processor means organized to respond to said signals to determine the tristimulus values of the color of the object in a reference colorimetric system using a transfer matrix to transform from the colorimetric system associated with said acquisition system into the reference colorimetric system, and a correction function for correcting the non-linearities of said acquisition system, said transfer matrix and said correction function being calculated using an iterative process based on the known tristimulus values in the reference colorimetric system of three primary colors and of at least two gray levels and from their tristimulus values in the colorimetric system associated with said acquisition system, as obtained by using said video camera to observe said primary colors and said gray levels.
In a preferred embodiment of the invention, the apparatus further includes a display system for reproducing all or a portion of the image observed by said video camera after said signals delivered by the acquisition system have been processed by said correction function.
Preferably, said display system includes a cathode ray tube (CRT) display device.
Advantageously, the apparatus further comprises a light source for illuminating the object placed in the field of observation of the camera, said source having a continuous emission spectrum I(xcex) selected so as to be close to a reference illuminant of spectrum D(xcex).
The camera includes a set of optical filters of spectra FR(xcex), FG(xcex), and FB(xcex) to resolve the image observed by the camera into primary color images on the sensors of said camera, advantageously the source is filtered by one or more filters for which the resultant filter function F(xcex) is selected so as to minimize the error of differences between the products D(xcex)xc2x7x(xcex), D(xcex)xc2x7y(xcex), and D(xcex)xc2x7z(xcex), and a linear combination of the products:
F(xcex)xc2x7I(xcex), FR(xcex)xc2x7F(xcex)I(xcex)xc2x7FG(xcex), and F(xcex)xc2x7I(xcex)xc2x7FB(xcex) where x(xcex), y(xcex), and z(xcex) are the spectral tristimulus values in the reference colorimetric system.
Preferably, said reference illuminant is the CIE illuminant of spectral D65(xcex).
The invention also provides a method of measuring the color of an object from an acquisition system including a video camera suitable for delivering signals representative of the tristimulus values, in a colorimetric system associated with the said acquisition system, of the color of an object placed in the field of observation of the camera, the method comprising the steps consisting in:
successively or simultaneously placing in the field of observation of the camera three primary colors and at least two gray levels, the tristimulus values of said primary colors and of said gray levels being known in a reference colorimetric system;
using an iterative process based on said tristimulus values in said reference colorimetric system and the corresponding tristimulus values in the colorimetric system associated with said acquisition system as obtained by observing said primary colors and said gray levels using said camera to calculate a transfer matrix for transforming from the colorimetric system associated with said acquisition system into the reference colorimetric system, and also a correction function for correcting the non-linearities of said acquisition system; and
determining the tristimulus values in the reference colorimetric system of the color of an object placed in the field of observation of the camera by using said transfer matrix and said correction function as calculated.
Advantageously, in the method, the color of the object is also viewed by means of a display system after the non-linearities of the acquisition system have been corrected.
Advantageously, in this method, the non-linearities of the display system are also corrected.
In a particular implementation of the method of the invention, the display system includes a CRT display device.
In a particular implementation of the method of the invention, a function for correcting the non-linearities of the display system is determined by:
displaying two zones having the same color but with luminances that may be different, the color of one of the zones being obtained by juxtaposing pixels having distinct control levels and the color of the other zone being obtained by a set of pixels all having the same control level corresponding to the mean of the control levels of the pixels of the other zone; and
making the luminances of the two zones equal for an observer by acting on pixel control level in one of the zones.
From the values of the pixel control levels in each of said zones before and after luminance equalization, information is deduced for calculating said correction function for correcting the non-linearities of the display system.
Preferably, one of the zones is rasterized.
Advantageously, said rasterized zone has every other raster line black.
When the display system includes a CRT display device and at a control level transition for pixels in the same raster line giving rise to a change in luminance between at least one pixel in said raster line and the pixel immediately following it, in the raster scanning direction, the control level for said immediately following pixel is advantageously selected as a function of the rate at which the control signal for the electron beam reaching pixels situated on the same raster line varies when the pixel control level varies.
To determine the correction to be made to take account of the rate at which the signal controlling the electron beam reaching pixels situated on the same raster line varies, it is advantageous to proceed as follows.
Two zones of the same color but of luminances that may be different are displayed, the color of one of the zones being obtained by juxtaposing on the same raster line pixels of different control levels, and the color of the other zone being obtained by a set of pixels all having the same control level, then the luminances of the two zones are made equal for an observer by acting on pixel luminance control level in one of the zones.
From the values of the pixel luminance control levels in each of said zones before and after equalization, information is deduced for calculating the correction to be provided in order to take into account the rate at which the control signal for the electron beam reaching the pixels situated in the same raster line varies when the pixel control level varies.
Preferably, the zone formed by Juxtaposing pixels of different luminances comprises, in the scan direction, alternating pixels each of luminance set to a level different from that of the preceding pixel.
The invention also provides a method of correcting the response of a display device having raster lines of pixels, in which, at a transition in the control level for pixels in the same raster line that gives rise to a variation of luminance at least between a pixel of said raster line and the pixel immediately following it in the raster scanning direction, the control level of said immediately following pixel is selected as a function of the rate at which the luminance of pixels situated on the same raster line varies when the control level of said pixels varies.
Advantageously, a correction function is determined for correcting the non-linearities of said display device by:
displaying two zones having the same color but luminances that may be different, the color of one of said zones being obtained by juxtaposing pixels having different control levels, while the color of the other zone is obtained by a set of pixels all having the same control level;
making the luminances of the two zones equal for an observer by acting on the pixel control levels of one of the zones; and
from the values of the pixel control levels of each of said zones, deducing information for calculating said correction function for correcting the non-linearities of the display device.
In a particular implementation of this method, said zone made up of pixels having different control levels is rasterized.
Advantageously, said rasterized zone includes raster lines in which every other raster line is black.
In another particular implementation of the method, said zone made up of pixels having different control levels includes alternating pixels in each raster line having a control level that is different from the control level of the preceding pixel in said raster line.
Also, in a particular implementation of the method of the invention, the above-mentioned iterative process comprises the steps consisting in:
calculating an approximate transfer matrix on the basis of the known tristimulus values of said primary colors and of said gray levels, and on the basis of an approximate correction function;
calculating a new approximate correction function using the approximate transfer matrix calculated in this way and the known tristimulus values of said gray levels, and by interpolating the missing values; and
recalculating the approximate transfer matrix and the approximate correction function until a fixed convergence threshold is reached.
In a variant, the iterative process comprises the steps consisting in:
calculating an approximate correction function on the basis of the known tristimulus values of said primary colors and of said gray levels, and on the basis of an approximate transfer matrix;
calculating a new approximate transfer matrix using the known tristimulus values of said grays and interpolating the missing values; and
recalculating the approximate correction function and the approximate transfer matrix until a fixed convergence threshold is reached.
Advantageously, non-uniformities in the illumination of the object by the source are also corrected, as are optical aberrations of the camera, by measuring the luminance of the screen at various points and by comparing it with the luminance at a reference point, e.g. the center of the image.